1. Field of the Invention
The present invention relates to a ceramic glow plug used for accelerating start-up of a diesel engine or a like application.
2. Description of the Related Art
FIG. 9 shows a ceramic glow plug 1 for use in a diesel engine. The ceramic glow plug 1 is configured in the following manner: a round-bar-like (columnar) ceramic heater 2 is fixedly fitted into the interior (cylindrical bore) of a straight tubular, metallic, cylindrical member 3 such that a heater end 2a projects from a front end 3a of the metallic, cylindrical member (hereinafter may be referred to as merely a cylindrical member) 3; and the resultant assembly consisting of the ceramic heater 2 and the cylindrical member 3 is fixedly fitted into (united with) a metallic, cylindrical plug body (hereinafter also referred to as merely a plug body or body) 4 such that the front end 3a of the cylindrical member 3 projects from a front end 4a of the body 4. The cylindrical member 3 used in this structure serves as a cover for protecting the ceramic heater 2 from various external forces (impact induced by dropping, a bending force applied in the course of mounting on an engine, etc.) that may be imposed on the ceramic heater 2 in the course of manufacture of the glow plug 1 and handling of the manufactured glow plug 1.
Such a ceramic glow plug 1 is usually assembled in the following manner. First, the ceramic heater 2 is loosely fitted into the cylindrical member 3, and a molten brazing metal (not shown) is poured into the gap therebetween. Thermal expansion of the cylindrical member 3 induced by the poured, molten brazing metal and contraction of the cylindrical member 3 induced by cooling are utilized for hermetically fixing the ceramic heater 2 in the cylindrical member 3 through squeezing action of the cylindrical member 3. Similarly, the assembly consisting of the ceramic heater 2 and the cylindrical member 3, together with other necessary components, is loosely fitted into the body 4, and a molten brazing metal is poured into the gap therebetween (brazing), thereby completing assembly. Instead of employing such brazing, the assembly consisting of the ceramic heater 2 and the cylindrical member 3 may be press-fitted into the body 4. However, this method is not put into practical use, because of a drawback described below.
As shown in FIGS. 9 and 10, the thus-assembled glow plug 1 is mounted onto a cylinder head 101 of a diesel engine via a threaded portion 6 formed on the outer circumferential surface of the body 4. Specifically, as shown in FIGS. 9 and 10, a mounting bore 104 for mounting the glow plug 1, the mounting bore 104 communicating with a prechamber 103, is provided on the cylinder head 101. The glow plug 1 is inserted from its end into the mounting bore 104. The threaded portion 6 of the body 4 is engaged with a threaded portion 105 formed in the mounting bore 104 such that the heater end 2a partially projects into the prechamber 103.
The mounting bore 104 is configured in the following manner. In order to mount the plug body 4 while a seal is established at the front end 4a of a cylindrical portion 7 of the plug body 4, a cylindrical bore 106 formed at a deep location of the mounting bore 104 allows the cylindrical portion 7 to be loosely fitted thereinto. Additionally, the front end 4a of the cylindrical portion 7 is located at a deep location of the cylindrical bore 106 and pressed against a dead end portion 107, whose bore diameter is smaller than the thread diameter. A cylindrical, small bore 108, whose diameter is equal to the bore diameter of the dead end portion 107, is formed at a location deeper than the dead end portion 107. The cylindrical member 3 is disposed in the cylindrical, small bore 108 in a loosely fit condition.
3. Problems to be Solved by the Invention
When current is applied to the thus-mounted glow plug 1 via electricity supply leads 15 and 16, a heating member embedded in the ceramic heater 2 generates heat through resistance to thereby accelerate start-up of an engine. The ceramic heater 2 is exposed to significant temperature variations (thermal shock) and blast waves during combustion of the engine. In the course of use, vibration and temperature rise may cause loosening at a joint between the cylindrical member 3 and the body 4. Such loosening potentially causes the cylindrical member 3 united with the ceramic heater 2 to drop off the inner circumferential surface of the body 4 into the prechamber 103.
Such a problem may arise even when the cylindrical member 3 united with the ceramic heater 2 is press-fitted into the body 4. When the cylindrical member 3 and the body 4 are to be joined together by press fitting, the cost of a glow plug can be lowered as compared with the case where the cylindrical member 3 and the body 4 are joined by brazing. This is because placement of a brazing metal and design to endure brazing temperature are not involved. In order to prevent breakage of the ceramic heater 2, which may result from stress concentration at the time of press fit, the tolerance of press fit must be reduced. However, a reduction in the tolerance of press fit increases the possibility of drop-off, and this is the reason why the press fit method is not put into practical use.
The present invention has been accomplished in view of the above-described problems involved in a conventional glow plug. It is therefore an object of the present invention to realize a structure for mounting a glow plug on a cylinder head, the structure preventing an assembly consisting of a cylindrical member and a ceramic heater from dropping off a body and entering an engine (prechamber), regardless of the means employed for joining the body and the cylindrical member, even when looseness arises in a joint between the body and the cylindrical member in the course of use of a glow plug mounted on the cylinder head.
The above object has been achieved in a first aspect of the invention by providing a ceramic glow plug in which a ceramic heater is fixedly disposed in a metallic, cylindrical member such that a front end thereof projects from a front end of the metallic, cylindrical member, and a resultant assembly consisting of the ceramic heater and the metallic, cylindrical member is fixedly disposed in a metallic, cylindrical plug body such that the front end of the metallic, cylindrical member projects from a front end of the metallic, cylindrical plug body.
The ceramic glow plug is characterized in that a portion of the metallic, cylindrical member which projects from the front end of the metallic, cylindrical plug body and extends over a predetermined range from the front end of the metallic, cylindrical member toward a rear end of the metallic, cylindrical member is smaller in outside diameter than a portion of the metallic, cylindrical member which is fixedly disposed within the metallic, cylindrical plug body, and that a front-end-oriented end face is formed in a diameter transition region of the metallic, cylindrical member where the outside diameter changes between those of the two portions.
According to the present invention, the above-described front-end-oriented end face is formed on the cylindrical member, thereby preventing entry of the cylindrical member into the cylinder head (prechamber) when the ceramic glow plug is mounted in a glow plug mounting bore (hereinafter also referred to merely as a mounting bore) formed in the cylinder head. Specifically, as described above, the ceramic glow plug is mounted onto the cylinder head via a threaded portion formed on an outer circumferential surface of the plug body so as to be screwed into a threaded portion of a ceramic glow plug mounting bore formed in the cylinder head until the front end of the plug body is pressed against a dead end portion (seat face) located at a deep location of the mounting bore and having a bore diameter smaller than a thread diameter of the threaded portion of the mounting bore. Therefore, in this mounting structure, entry of the cylindrical member into the cylinder head can be prevented in the following manner: movement prevention means is provided in the mounting bore for preventing movement of the front-end-oriented end face deep into the mounting bore so as to prevent the metallic, cylindrical member from moving deep into the mounting bore and into the interior of the cylinder head. That is, in the present invention, a structural feature may be formed in the mounting bore which the front-end-oriented end face abuts so as to prevent entry of the metallic, cylindrical member into the interior of the prechamber. Thus, the present invention can easily and reliably prevent a serious accident, such as an accident arising conventionally in which the cylindrical member that holds the ceramic heater separates from the body and drops into the cylinder head due to loosening at the joint between the body and the cylindrical member.
A portion of the metallic, cylindrical member which projects from the front end of the metallic, cylindrical plug body and is smaller in outside diameter than a portion of the metallic, cylindrical member fixedly disposed within the metallic, cylindrical plug body may extend over a predetermined range from the front end of the metallic, cylindrical member toward a rear end of the metallic, cylindrical member. Therefore, the small-diameter portion of the metallic, cylindrical member may be the entire portion which projects from the front end of the metallic, cylindrical plug body, or may be most of or part of the projecting portion which extends from the front end toward the rear end. Preferably, in the above-described means to solve the problems of the prior art, as described in a second aspect of the invention, the front-end-oriented end face formed in the diameter transition region of the metallic, cylindrical member and the front end of the metallic, cylindrical plug body are substantially aligned with each other in the axial direction of the ceramic heater. In this manner, when the small-diameter portion of the metallic, cylindrical member is substantially the entire portion which projects from the front end of the metallic, cylindrical plug body, the movement prevention means for preventing movement of the front-end-oriented end face deep into the mounting bore can be implemented by rendering the bore diameter of the dead end portion of the mounting bore smaller than the outside diameter of the large-diameter portion having a relatively large diameter and being located on the side toward the rear end with respect to the front-end-oriented end face, thereby avoiding a complex mounting structure. Preferably, when the front-end-oriented end face is to be substantially aligned with the front end of the metallic, cylindrical plug body, the front-end-oriented end face is shifted slightly toward the rear end from the front end of the plug body. As a result, by rendering the bore diameter of the dead end portion smaller than the outside diameter of the large-diameter portion of the cylindrical member, a seal is established at the interface between the end of the body and the dead end portion.
A third aspect of the present invention is a ceramic glow plug described in the above first or second aspects, the ceramic glow plug being characterized in that the difference in outside diameter between a small-diameter portion of the metallic, cylindrical member and a large-diameter portion of the metallic, cylindrical member is not less than 0.3 mm, the small-diameter portion having a relatively small diameter and being located on a side toward the front end with respect to the front-end-oriented end face, which is formed in the diameter transition region of the metallic, cylindrical member, and the large-diameter portion having a relatively large diameter and being located on a side toward the rear end with respect to front-end-oriented end face.
Employing a difference of not less than 0.3 mm in outside diameter between the large-diameter portion and the small-diameter portion as described in the above third aspect facilitates design of the movement prevention means for preventing movement of the front-end-oriented end face deep into the mounting bore. In view of reliable prevention of movement, a greater difference in diameter is preferred. However, an excessively great difference in diameter is accordingly accompanied by an increase in the wall thickness of the large-diameter portion of the cylindrical member or decrease in the wall thickness of the small-diameter portion. In view of these drawbacks, an appropriate difference in diameter is 0.5-2 mm.
In the above-described means for solving the problems of the prior art, the metallic, cylindrical member may be press-fitted into the metallic, cylindrical plug body as described in a fourth aspect of the invention, or the metallic, cylindrical member may be fixedly disposed in the metallic, cylindrical plug body via a brazing metal (hereinafter this process is also referred to as brazing) as described in a fifth aspect of the invention. Because of use in the aforementioned severe environment, a conventional glow plug may suffer separation of the cylindrical member and the body even when either fixation structure is employed. The present invention can easily solve these problems when either fixation structure is employed. Particularly, the present invention allows practical use of fixation by press fitting, which has conventionally encountered difficulty in being put into practical use. This is because dimensional tolerance of the press fit can be alleviated, thereby realizing a low-cost glow plug.
A structure for mounting a ceramic glow plug on a cylinder head as described in a sixth aspect of the invention is configured in the following manner. A ceramic glow plug-in which a ceramic heater is fixedly disposed in a metallic, cylindrical member such that a front end thereof projects from a front end of the metallic, cylindrical member, and a resultant assembly consisting of the ceramic heater and the metallic, cylindrical member is fixedly disposed in a metallic, cylindrical plug body such that the front end of the metallic, cylindrical member projects from a front end of the metallic, cylindrical plug body-is mounted onto a cylinder head via a threaded portion formed on an outer circumferential surface of the metallic, cylindrical plug body so as to be screwed into a threaded portion of a ceramic glow plug mounting bore formed in the cylinder head until the front end of the metallic, cylindrical plug body is pressed against a dead end portion located at a deep location of the mounting bore and having a bore diameter smaller than a thread diameter of the threaded portion of the mounting bore.
The structure for mounting a ceramic glow plug on a cylinder head is characterized in that a portion of the metallic, cylindrical member which projects from the front end of the metallic, cylindrical plug body and extends over a predetermined range from the front end of the metallic, cylindrical member toward a rear end of the metallic, cylindrical member is smaller in outside diameter than a portion of the metallic, cylindrical member which is fixedly disposed within the metallic, cylindrical plug body, and a front-end-oriented end face is formed in a diameter transition region of the metallic, cylindrical member where the outside diameter changes between those of the two portions.
The structure for mounting a ceramic glow plug on a cylinder head is further characterized in that movement prevention means is provided in the mounting bore for preventing movement of the front-end-oriented end face deep into the mounting bore so as to prevent the metallic, cylindrical member from moving deep into the mounting bore and into interior of the cylinder head.
A structure for mounting a ceramic glow plug on a cylinder head as described in a seventh aspect of the invention is configured such that, in the sixth aspect, the movement prevention means is implemented by rendering the bore diameter of the dead end portion of the mounting bore smaller than an outside diameter of a large-diameter portion having a relatively large diameter and being located on a side toward the rear end with respect to the front-end-oriented end face.
A structure for mounting a ceramic glow plug on a cylinder head as described in an eighth aspect of the invention is configured such that, in the sixth aspect, the movement prevention means is implemented by rendering a diameter of the mounting bore as measured at a location located deeper than the dead end portion smaller than an outside diameter of a large-diameter portion having a relatively large diameter and being located on a side toward the rear end with respect to the front-end-oriented end face.